Kelim, Chapter Twenty-Four, Mishnah Fourteen
There are three different types of napkins:
1) One used for hands is susceptible to midras uncleanness;
2) One used for books is susceptible to corpse uncleanness;
3) And the one used as a shroud or a covering for the harps of the Levites is free from all uncleanness.
Section one: The word that I have translated here as “napkins” really would be best translated as a small piece of cloth. If the cloth is used to clean one’s hands, it is susceptible even to midras impurity because it is sometimes used to cover the seat cushion and then it is sat upon.
Section two: If it is used to cover a book, it counts as a vessel and therefore it is susceptible to impurity.
Section three: If the cloth is used only to cover a dead body or to cover a harp it is not considered a vessel. The difference between this and a book cover is that one continues to use a book when it is covered, whereas one cannot use a harp (or a dead body for that matter) when it is covered.